Over the last few years there has been a great deal of work on single pass Josephson junction Analog to Digital (A/D) converters using two- and three-junction Superconducting QUantum Interference Devices (SQUIDs) (see, for example, C. A. Hamilton et al., "Superconducting A/D Converter Using Latching Comparators", IEEE Tran. Magn., vol MAG-21, pp. 197-199, Mar. 1985 and Spargo et al, "Analog Signal Processing With Josephson Junctions: Analog to Digital Conversion", Extended Abstracts of the 1987 International Superconductivity Electronics Conference, Aug. 28, 1987, Tokyo, pp. 319-334). Single pass A/D converters are attractive because of their increased speed of analog-to-digital conversion compared to multiple pass converters such as successive approximation and parallel feed-forward converters. These single pass A/D converters typically use two- and three-junction converters of the type illustrated in these two references.
As taught in Spargo, contrary to conventional A/D converters which require 2.sup.n -1 comparators to distinguish the 2.sup.n levels characteristic of an n-bit A/D converter, an A/D converter utilizing periodic threshold comparators can distinguish these 2.sup.n levels with n periodic threshold comparators. Such a comparator is illustrated in FIG. 1.
Unfortunately, as taught by Hamilton et al, these A/D converters exhibit a 100 MHz bandwidth that is far less than the 5 gigasamples/second potential conversion rate of the converters. The reasons for this are as follows. In a single pass converter, a gate signal is used to capture the value of the test signal during the gate pulse. In order to divide the range of the test signal into 2.sup.n levels, the gate pulse must be narrow enough that the signal under test does not vary by more than I.sub.max /2.sup.n during the gate pulse. This limits the frequency response of the A/D converter to less than 1/2.sup.n .pi..tau. where .tau. is the width of the gate pulse. Hamilton shows that, in an 8-bit A/D converter utilizing prior art two- and three-junction SQUIDs (such as the two-junction SQUID of FIG. 2), when the frequency of the signal under test is above 100 MHz, the gate signal produces hysteretic transient responses 31 and 32 (shown in FIG. 3) that cause spurious A/D conversion. These transients occur at those values of the test signal I.sub.a at which the quantized magnetic flux through the SQUID changes abruptly between successive discrete values. These transient responses interfere with the less significant bits of the A/D converter and introduce hysteretic effects that make the measured value dependent on whether the signal is increasing or decreasing. Thus, to achieve the potential conversion speed of A/D converters utilizing SQUIDs, a new design of the SQUID comparator is needed.